This invention relates to zirconia-containing ceramic compositions for thermal barrier coatings having reduced thermal conductivity that comprise a stabilizer metal oxide selected from ytterbia, neodymia or mixture thereof, with or without lanthana, to stabilize the zirconia in the cubic crystalline phase. This invention also relates to ceramic compositions comprising zirconia and these stabilizer metal oxides that additionally comprise hafnia for balancing the conductivity of the resultant coating, and/or tantala to contribute additional conductivity reductions. This invention further relates to coatings prepared from such compositions, articles having such coatings and methods for preparing such coatings for the article.
Components operating in the gas path environment of gas turbine engines are typically subjected to significant temperature extremes and degradation by oxidizing and corrosive environments. Environmental coatings and especially thermal barrier coatings are an important element in current and future gas turbine engine designs, as well as other articles that are expected to operate at or be exposed to high temperatures, and thus cause the thermal barrier coating to be subjected to high surface temperatures. Examples of turbine engine parts and components for which such thermal barrier coatings are desirable include turbine airfoils such as blades and vanes, turbine shrouds, combustion liners, deflectors, and the like. These thermal barrier coatings typically comprise the external portion or surface of these components and are usually deposited onto a metal substrate (or more typically onto a bond coat layer on the metal substrate for better adherence) from which the part or component is formed to reduce heat flow (i.e., provide thermal insulation) and to limit (reduce) the operating temperature the underlying metal substrate of these parts and components is subjected to. This metal substrate typically comprises a metal alloy such as a nickel, cobalt, and/or iron based alloy (e.g., a high temperature superalloy).
For reduced thermal conductivity, the thermal barrier coating is usually prepared from a ceramic material, such as zirconia. It is also desirable to chemically phase-stabilize the zirconia-containing thermal barrier coating in the cubic phase. The cubic phase is desirable for lower thermal conductivity and can be either fluorite or pyrochlore structure. Examples of such chemically phase-stabilized zirconias include yttria-stabilized zirconia, scandia-stabilized zirconia, ceria-stabilized zirconia, calcia-stabilized zirconia, and magnesia-stabilized zirconia. The thermal barrier coating of choice is typically a yttria-stabilized zirconia ceramic coating. A representative yttria-stabilized zirconia thermal barrier coating usually comprises about 7 weight % yttria and about 93 weight % zirconia.
There are a variety of ways to further reduce the thermal conductivity of such thermal barrier coatings. One is to increase the thickness of the coating. However, thicker thermal barrier coatings suffer from weight and cost concerns. Another approach is to reduce the inherent thermal conductivity of the coating. One effective way to do this is to provide a layered structure such as is found in thermal sprayed coatings, e.g., air plasma spraying coatings. However, coatings formed by physical vapor deposition (PVD), such as electron beam physical vapor deposition (EB-PVD), that have a columnar structure are typically more suitable for turbine airfoil applications (e.g., blades and vanes) to provide strain tolerant, as well as erosion and impact resistant coatings.
Another general approach is to make compositional changes to the zirconia-containing ceramic composition used to form the thermal barrier coating. A variety of theories guide these approaches, such as: (1) alloying the zirconia lattice with other metal oxides to introduce phonon scattering defects, or at higher concentration levels, to provide very complex crystal structures; (2) providing “coloring agents” that absorb radiated energy; and (3) controlling the porosity and morphology of the coating. All of these approaches have limitations. For example, modifying the zirconia lattice, and in particular achieving a complex crystal structure, limits the potential options for chemical modification and can interfere with good spallation resistance and particle erosion resistance of the thermal barrier coating.
Accordingly, it would be desirable to be able to change the compositional parameters of the zirconia-containing ceramic composition to achieve further reductions in the thermal conductivity of the resultant thermal barrier coating, yet still be able to stabilize the zirconia in the cubic crystalline phase. It would further be desirable to be able stabilize the zirconia in the cubic crystalline phase without needing to include traditional phase stabilizers for zirconia such as yttria.